16. High Performance Considerations

Depending on your hardware, Sagan can operate comfortably up to about 5k “events per/second” (EPS) using default configurations. When you hit this level and higher, there are a few configuration options to take into consideration.

16.1. batch-size

The most important thing is the batch-size sagan.yaml configuration option. By default, when Sagan receives a log line, the data is sent to any available thread. Due to memory protections (pthread mutex lock/unlock), this isn’t efficient. The system starts to spend more time protecting the memory location of the single line of log data than processing the log line.

The batch-size allows Sagan to send more data to worker threads and use less “locks”. For example, with a batch-size of 10, Sagan can send 10 times more data with only one “lock” being applied. At even higher rates, you may want to consider setting the batch-size to 100.

The default batch sizes are 1 to 100. On very high performance systems (100k+ EPS or more), you may want to consider rebuilding to handleeven larger batches. To do this, you would edit the sagan-defs.h and change the following.

#define MAX_SYSLOG_BATCH        100


#define MAX_SYSLOG_BATCH        1000

Then rebuild Sagan and set your batch-size to 1000. While you will save CPU, Sagan will use more memory. If you sent the MAX_SYSLOG_BATCH to 1000 and only set the batch-size to 100, Sagan will still allocate memory for 1000 log lines. In fact, it will do the per-thread! Think of it this way:

( MAX_SYSLOG_BATCH * 10240 bytes ) * Threads = Total memory usage.

The default allocation per log line is 10240 bytes.

16.2. Rule sets

At high rates, consideration should be given to the rules that you are loading. Unneeded and unused rules waste CPU.

If you are writing rules, make sure you use simple rule keywords first (content, meta_content, program, etc) before moving to more complex rule options like pcre. The more simple rule keywords can be used to “short circuit” a rule before it has to do more complex operations.

Software like Snort attempts to arrange the rule set in memory to be more efficient. For example, when Snort detects multiple content modifiers, it shifts the shortest lenght content to the front (first searched). Regardless of the content rule keywords placement within a rule.

Because logs are inherently different than packets, Sagan does not do this! If you have multiple content keywords, Sagan will use them in the order they are placed in the rule. You will want to use the least matched keywords as the first content. For example:


# This will use more CPU because "login" is common.

content: "login"; content: "mary";

# This will use less CPU because "mary" is likely less common.

content: "mary"; content: "login";

The same login applied to pcre and meta_content.

16.3. Rule order of execution

Sagan attempts to use the least CPU intensive rule options first. This means that if a Sagan rule has multiple content keywords and multiple pcre keywords, the content rule keywords are processed first. If the content keywords do not match, then there is no need to process the pcre keywords. The order of execution within a rule is as follows:

The program field is the very first thing to be evaluated.

The content is the next option Sagan takes into consideration.

The meta_content is next.

Finally the pcre option, which is consided the heaviest, is the last.